Multidrug resistance is a major obstacle to curing cancer because cancer cells become resistant to diverse and unrelated therapeutic compounds. A mechanism for multidrug resistance is the active extrusion of chemotherapeutic drugs from cancer cells by ABC transporters. ABCG2 is a promiscuous ABC transporter of many unrelated compounds. Mutant forms of ABCG2 are expressed in elevated levels in multidrug resistant cancers of diverse origins including fibroblasts, breast, colon, lung, and ovaries. The mechanisms of drug transport remain unclear and the functions of each domain of ABCG2 are neither tested nor confirmed. Furthermore, there are no 3-dimensional structures of ABCG2. The following specific aims are proposed to rectify this situation: 1) To characterize full length ABCG2 and its domains. The activity of each domain of ABCG2 will be tested using assays that measure cytotoxicity, ATPase activity, drug binding and drug extrusion. 2) To determine 3-dimensional structures of ABCG2's cytosolic domain. X-ray structures will be solved that reveal the mode of binding of nucleotides to the cytosolic domain. 3) To determine 3-dimensional structures of full length ABCG2 and domains. Structures will be solved of full length and active domains of ABCG2. The correlation of structural and activity data will clarify the mechanism of drug transport by ABCG2 and homologous transporters, thus spearheading new strategies for the development of novel chemotherapies for multidrug resistant cancer. Specific aims 1 and 2 are proposed for Phase I, while specific aim 3 is proposed for Phase II of the K01 award and beyond. The K01 will afford the applicant protected time to develop new skills and to apply existing skills to cancer research, with the guidance of her mentors, at the stimulating educational environment of the Eppley Institute. She will then be ready to successfully compete for and obtain an independent tenure track position in cancer research.